The present invention relates to a hydraulic control system, which system can modulate the characteristic of hydraulic fluid pressure supplied to a shifting friction element.
Referring to FIG. 7, a known hydraulic control system for an automatic transmission is described. FIG. 7 shows in diagram a high clutch H/C, a low clutch L/C, a band servo B/S of a dual piston type movable to a brake apply position when a servo apply chamber S/A is pressurized with a servo release chamber S/R depressurized or to a brake release position when the servo release chamber S/R is pressurized regardless of whether the servo apply chamber S/A is pressurized or not. The hydraulic control system also comprises a 1-2 shift valve 1000, a 2-3 shift valve 1001, and a 3-4 shift valve 1002. Reference numeral 1003 designated a D range pressure passage, reference numeral 1004 designates a low clutch pressure passage, reference numeral 1005 designates a D2-D4 speed pressure passage, reference numeral 1006 designates a D3-D4 speed passage, reference numeral 1007 designates a high clutch pressure passage, and reference numeral 1008 designates a servo release pressure passage. Shift shock reducing arrangements include orifices 1009, 1010, 1011, 1012, 1013, and 1014, and accumulators 1015 and 1016.
The D range pressure passage 1003 and low clutch pressure passage 1004 play a role in forming a hydraulic fluid passage for establishing the first speed within D range, and the D2-D4 speed pressure passage 1005 also plays a role in forming a hydraulic fluid passage for establishing the second speed within D range, the D3-D4 speed pressure passage 1006, high clutch pressure passage 1007 and servo release pressure passage 1008 also play a role in forming a hydraulic fluid passage for establishing the third speed within D range, and the hydraulic pressure passage for the third speed within D range minus the low clutch pressure passage 1004 and servo release pressure passage 1008 provides a hydraulic fluid passage for the fourth speed within D range.
In selecting D range first speed from N range, the orifices 1009 and low clutch accumulator 1015 play an important role in alleviating shock upon engagement of the low clutch L/C. In shifting from D range first speed to D range second speed, the orifice 1014 and band servo accumulator 1016 play a role in allevating shock upon application of the band brake B/B by the band servo B/S. In shifting from D range second speed to D range third speed, the supply of fluid pressure to the high clutch and that to the servo release chamber S/R are synchronized by the common orifice 1011 and the occurrence of shift shock is alleviated by the orifices 1011 and 1013. In shifting from D range third speed to D range fourth speed, the discharge of hydraulic fluid from the low clutch L/C and that from the servo release chamber S/R are synchronized by the common orifice 1010. The reference numeral 1017 designates a servo release timing valve which connects the servo release chamber S/R to the high clutch H/C or the low clutch L/C, selectively, in order to effect shifting between the first speed, third speed and fourth speed.
The conventional control system uses orifices to restrict fluid flow and accumulators to modulate rising characteristics of hydraulic fluid pressure. If the specifications of the orifices and accumulators are set, a pattern in which servo actuating hydraulic pressure rises is fixed accordingly and is invariable. Thus, it cannot satisfactorily comply with varying requirements in the case where the torque capacity demand on the same shifting element differs from one to another speed position or in the case where the engine torque varies.
An object of the present invention is to solve the above mentioned problem and provide a hydraulic control system which can easily modulate the characteristic of hydraulic fluid pressure supplied to a shifting friction element.